Terme Achilliane, Ancient Roman thermal baths in Piazza del Duomo, Italy.

Terme Achilliane is an ancient Roman bathing complex situated beneath Piazza del Duomo, dating from around the 4th century and now integrated under the later Cathedral of Sant'Agata. The main chamber is supported by four square pillars and features three arched passageways that open into parallel water basins.

The founding date is debated: a 4th-century origin is likely, and Roger Wilson suggested the 3rd century. A long inscription records that around 434 the building was reduced to save firewood for heating. In 1088 Bishop Ansgerio chose the site for the Cathedral of Sant'Agata and its Benedictine monastery, completed in 1094.

The name comes from an inscription on Lunense marble from the 5th century, now displayed at the Civic Museum in Castello Ursino. This detail connects the ancient bathing tradition to the city's broader story, preserved through artifacts in the nearby museum.

Entry is through a barrel-vaulted corridor between the cathedral foundations, accessible via a descending ramp to the right of the facade. The site requires careful footing on uneven ground, so sturdy shoes are advisable when exploring the underground passages.

The visible chamber is probably one of the frigidaria, not the heated Tepidarium, which lay off the left entrance and was warmed by channelled air and linked to the upper floor by a two-ramp staircase. Near the corridor's end, deep excavations revealed an S-shaped channel feeding the pillared hall, likely for water distribution.